chai-as-promised

Chai Assertions for Working with Promises

Chai as Promised extends Chai with a fluent language for asserting facts about promises.

Instead of manually wiring up your expectations to a promise's fulfilled and rejected handlers:

doSomethingAsync().then(
    function (result) {
        result.should.equal("foo");
        done();
    },
    function (err) {
       done(err);
    }
);

you can write code that expresses what you really mean:

doSomethingAsync().should.eventually.equal("foo").then(done, done);

or if you have a testing framework that follows the UncommonJS specification for handling promises, simply

return doSomethingAsync().should.eventually.equal("foo");

How to Use

The most powerful extension provided by Chai as Promised is the eventually property. With it, you can transform any existing Chai assertion into one that acts on a promise:

(2 + 2).should.equal(4);

// becomes
return promiseFor(2 + 2).should.eventually.equal(4);

({ foo: "bar" }).should.have.property("foo");

// becomes
return promiseFor({ foo: "bar" }).should.eventually.have.property("foo");

There are also a few promise-specific extensions, grouped here as synonymic blocks:

return promise.should.be.fulfilled;

return promise.should.be.rejected;
return promise.should.be.broken;

return promise.should.eventually.eql("foo");
return promise.should.become("foo");

return promise.should.be.rejected.with(Error);
return promise.should.be.broken.with(Error);

// Note: other variants of Chai's existing `throw` assertion work too.

Installation and Usage

Node

Do an npm install chai-as-promised to get up and running. Then:

var chai = require("chai");
var chaiAsPromised = require("chai-as-promised");

chai.use(chaiAsPromised);

You can of course put this code in a common test fixture file; for an example using [Mocha][mocha], see the Chai as Promised tests themselves.

AMD

Chai as Promised supports being used as an AMD module, registering itself anonymously (just like Chai). So, assuming you have configured your loader to map the Chai and Chai as Promised files to the respective module IDs "chai" and "chai-as-promised", you can use them as follows:

define(function (require, exports, module) {
    var chai = require("chai");
    var chaiAsPromised = require("chai-as-promised");

    chai.use(chaiAsPromised);
});

<script> tag

If you include Chai as Promised directly with a <script> tag, it creates a window.chaiAsPromised global (again, just like Chai). Then your setup code becomes:

window.chai.use(window.chaiAsPromised);